1. Field of the Invention
The present invention relates to a bolometer type uncooled infrared ray sensor and its driving method.
2. Related Art
Image sensors utilizing infrared rays have features that they can pick up images night and day and the infrared rays are higher in permeability through smoke or fog than visible light. In addition, the image sensors utilizing infrared rays have a feature that temperature information of a subject can also be obtained. Therefore, the image sensors utilizing infrared rays have a wide application range as surveillance cameras and fire sensing cameras not to speak of the defense field.
A greatest drawback of quantum type infrared ray solid image pickup devices which are conventional mainstream devices is that a cooling mechanism for low temperature operation is needed. In recent years, however, uncooled infrared ray image sensors which do not need such a cooling mechanism have been developed (see, for example, Japanese Patent No. 3793033).
However, it can be said that the infrared ray image sensors are insufficient in sensitivity and dynamic range as compared with image sensors utilizing visible rays. For example, if it is attempted to use the infrared ray image sensors in surveillance camera, sensors having a NETD (Noise Equivalent Temperature Difference) of approximately 0.1 K and a dynamic range of approximately 30 K are needed. Development of a sensor having enhanced output linearity, a higher sensitivity and a wider dynamic range is anticipated.
On the other hand, noise generated in image sensors can be broadly classified into random noise and fixed pattern noise. As the random noise, there are 1/f noise having a power spectrum which is in inverse proportion to the frequency f and kTC noise which is thermal noise caused by capacitance. The fixed pattern noise is caused mainly by characteristic variations of diodes and threshold variations of transistors.
For reducing such noise, the noise can be canceled in principle by finding a difference between a signal voltage and a reference signal before and after resetting. Such a correlated double sampling (CDS) circuit with an electrical reset switch is typically used in visible region image sensors using photodiodes. In the case of thermal sensors using a bolometer, a thermal reset mechanism must be fabricated, making the CDS circuit unpractical.
Although the fixed pattern noise and 1/f noise are dominant in conventional bolometer type uncooled infrared ray sensors as described above, there are no effective means, resulting in a problem.